Sputter coating is a process carried out in a vacuum chamber, filled with a generally chemically inert gas, in which a substrate to be coated is mounted facing a target formed of the coating material. In the chamber, the target is subjected to an electrical potential negative with respect to the chamber wall or some other anode within the chamber. The potential gradient adjacent the target surface causes electrons to be emitted from the target. As they are attracted toward the chamber anode, the emitted electrons strike and ionize some of the atoms of the inert gas by stripping electrons from them. Positive ions of the gas are thereby formed and attracted toward the negative target which they strike, transferring momentum to the target material, and ejecting particles of the material from the target surface. The substrate to be coated, which is positioned in the chamber usually with its surface facing the target so as to intercept the moving particles of coating material sputtered from the target, receives some of the ejected particles, which adhere to and coat the substrate surface.
In magnetron enhanced sputtering processes, a magnetic field is formed over the target surface with magnetic field lines having components extending parallel to the target surface. In many applications, the field lines arch over the target surface and form a closed magnetic tunnel. The magnetic field causes the electrons moving from the target to curve in spiral paths over regions of the target surface enclosed by the field, thereby increasing the electron density in the enclosed space, and resulting in an increase in the rate of electron collisions with gas atoms over the enclosed regions of the target surface. The increased collision rate in turn increases the ionization of the gas in the enclosed space and thus increases the efficiency of the sputtering process at the underlying target region. Where the magnetic field lines equal or exceed a critical field strength over the target surface, a glowing ion cloud or plasma is seen trapped within the field over the region of the target surface.
In the commonly assigned and copending U.S. patent application Ser. No. 07/339,308, filed Apr. 17, 1989, now U.S. Pat. No. 4,957,605, entitled "Method and Apparatus for Sputter Coating Stepped Wafers," expressly incorporated herein by reference, a sputter coating apparatus and method are disclosed in which a concave annular target is provided with a pair of concentric annular electromagnets with concentric pole pieces behind and at the rim of the target. The fields produced by these magnets cause the formation of a pair of concentric plasma rings overlying concentric sputtering regions on the target surface. The two plasma rings are alternately energized by alternately supplying current to the magnet coils while the target power is switched between to controlled power levels in synchronization with the switching of the current to the magnetic coils. This causes the two target regions to be alternately activated so that the sputtering from the regions is alternately switched on and off. This provides different controllable rates of sputtering from inner and outer concentric regions of the surface of a single piece sputtering target.
Separate control of the sputtering from the plural target regions enables the control of the distribution characteristics of the sputtered material deposited on the substrate or wafer being coated. For example, varying the relative parameters affecting the energization of the two target regions, as for example the "on" power levels or the duty cycle of the activation of each target region, provides control of coating uniformity on the substrate surfaces. This control is especially important where differently facing surfaces of substrates such as stepped semiconductor wafers must be uniformly coated. The aforereferenced patent application Ser. No. 07/339,308 particularly describes in detail certain effects on the coating uniformity caused by target and substrate geometry and by the electrical parameters relating to the energization of the target and the plasmas. The application also discusses the effects of target erosion on sputter coating uniformity.
By its very nature, the process of cathode sputter coating involves the removal of material from the sputtering target and the redeposition of the sputtered material onto the substrate surface. The removal of material from the cathode sputtering target consumes the target, reducing the thickness of the target until eventually an erosion groove or area will "punch through" to the back surface of the target. The erosion of the target surface is usually uneven, being concentrated in areas which underlie the denser regions of ion concentration or plasmas in the space above the target adjacent the target surface. To broaden the area of target erosion, some prior art devices have caused the plasma to move on the target surface, usually by movements made in magnetic fields. This movement of the plasma moves the area of erosion about the surface of the target reducing the tendency of a sharp erosion groove to be formed. Movement of the position of the plasmas, however, incapacitates, or at least complicates, the selective control of the sputtering rate from different target regions to achieve coating uniformity.
With magnetron sputtering devices, the plasmas are generally confined to one or more regions of a target surface, in part due to design requirements of the magnet structure, and in part due to certain performance requirements which necessitate the location of the plasmas in specific geometric positions in relation to the substrate surfaces to achieve a desired coating distribution on the substrate. For example, in U.S. patent application Ser. No. 07/339,308, expressly incorporated by reference above, the maintenance of separate plasmas on a target in specific geometric relationships with the substrate surface are employed in order to control the uniformity of the coating on a substrate surface, particularly where the surface of the substrate includes diversely facing surfaces such as steps on semiconductor wafers.
In the above-referenced patent application, the positions of the plasmas determine the locations from which the sputtering material is emitted, which determines the corresponding distribution of the deposited coating material on the substrate surface. By controlling the ratios of material emitted from different sputtering regions on a sputtering target, the uniformity of the coating is controllable. Accordingly, it is important that the location of the sputtering regions be located on the target in particular positions selected to provide the desired coating uniformity. Thus, the techniques devised by certain devices of the prior art for moving the plasmas about the target surface in order to redefine the areas of erosion throughout the life of the target interfere with the ability to freely achieve desired coating uniformity by precise placement of the plasma and, consequently, of the erosion region of the target.
Erosion of the target surface by the emission of sputtering material is manifested in the formation of a progressively deepening erosion groove. The formation of this erosion groove alters the performance of the sputtering target, generally with a delaying emission rate from the sputtering target region, a phenomenon referred to as rate "roll-off". This rate roll-off is due in part to the fact that the erosion groove is receding geometrically from the substrate surface, but more significantly, is due to the change in contour of the target surface and the deeply steepening sides of the erosion groove. The steepened sides of the erosion groove tend to shift the direction of emission of the flux of sputtering material, causing it to be less predominantly directed toward a substrate to be coated. In addition, the redirection of sputtered material tends to cause impingement of the material on the oppositely facing wall of the erosion groove and a redeposition of the material onto the target surface. Accordingly, while this erosion proceeds, redeposition of material on the side walls of the erosion groove tends to further narrow the groove. Also, in that sputtering energy is consumed by emission of material, redeposition of the sputtered material onto the target, rather than onto the substrate, progressively lessens the efficiency of the process of effectively coating the substrate surface. Thus, a decline in the deposition rate is experienced. Compensation for the effect of a declining deposition rate is usually achieved by progressively increasing the power applied to the target over the course of the useful target life in order to maintain an acceptable or even constant deposition rate onto the substrates.
The deepening of the steep erosion groove throughout the life of the target and the corresponding necessitated increase in sputtering power have certain disadvantages which shorten the life of the target and inhibit the use of the material of the target efficiently. The deepening of the erosion groove tends to progress toward a rapid punch through of the target in a small area or band on the target surface. When this occurs, the remainder of the material in the target can no longer be used, as the target's life has ended. In addition, the continual increase of the power of the target in order to provide an effective deposition rate, in many cases, will exceed the maximum power which the target can handle and, accordingly, the target life may be prematurely ended when the target can no longer be energized to operate at an efficient sputtering rate. Limiting the increased power to a safe power tends to unacceptably slow down the sputtering process which may have altered effects on the quality of the substrate coating being applied and in addition render the use of the equipment inefficient.
The location of the erosion groove is determined by the placement, in a magnetron sputtering apparatus, of magnet structure which includes pole pieces positioned either behind the side or around portions of the sputtering target. The magnets so formed usually generate magnetic fields which arch over the sputtering regions of the target and which confine the ion producing plasmas therein. The magnetic field lines which over the target generally decline in strength with the distance from the magnet. In order for such fields to effectively confine a plasma, it is necessary that some field line of a particular minimum critical strength arch over the target surface. The necessary strength for the critical field line is dependent on several design parameters, but may, for example, be in the area of approximately 160 to 180 gauss. When a target is new and its sputtering surface is farthest from the magnet, it is found that a certain amount of magnetism is required to produce a critical field line of a proper size and shape over the desired location on the target surface to effectively confine a plasma. Where the magnets are electromagnets and the strengths of the magnetic fields relate to the level of current through the magnet windings, the critical field strength with such a new target can be precisely established.
As the target erodes, however, the erosion groove is formed and the surface of the target recedes toward the underlying magnet. If the strength and shape of the magnetic field are constant as the target erodes, the changing contour of the target surface causes the surface to erode fastest where the magnetic field is strongest and the lines of the strongest field bridge the target surface. Therefore, where the magnetic field at the center of an erosion zone of a new target may have been in the area of 180-190 gauss, as the erosion groove is formed, the field strength at the target surface within the erosion groove may increase to, for example, 240 gauss. At this field strength increases at the target surface, the plasma which forms tends to be more tightly confined and drawn more closely to the target surface. This is found to occur at the center of the erosion groove. This tightening and compacting of the plasma in the presence of the increased field strength is believed to accelerate the formation of a sharply defined, deep, narrow erosion groove in the target surface. While the formation of the deep steeply walled narrow erosion groove in the target surface may be partially overcome with the prior art proposal to move the magnetic field and thus the position of the plasma, in many cases, this will shift the point of origin of the sputtering material so as to unacceptably alter the uniformity of the coating on the substrate surface.
While many schemes for controlling the electrical parameters of a target have been devised, these control schemes have focused on satisfying parameters such as substrate coating uniformity. The prior art has not, however, effectively produced a method for controlling the operating parameters of the sputtering apparatus over the course of the life of the sputtering target in such a way as to avoid undesirable formation of the steep erosion groove without sacrificing or limiting the ability to otherwise control the sputter coating process. It is desirable that the formation of the erosion groove be controlled in such a way that the contour or profile of the target surface most closely conforms to that of the original target throughout the target life, and particularly to be able to do so without moving the sputtering region on the target surface. In this way, the efforts to control the uniformity of the deposition on substrates is rendered easier to achieve while the use of the target material is made more efficient.